Unifying the mechanisms for alkane dehydrogenation and alkene H/D exchange with [IrH_2(O_2CCF_3)(PAr_3)_2]: the key role of CF_3CO_2 in the 'sticky' alkane route

摘要

To understand photochemical and thermal alkane activation with IrH_2(O_2CCF_3)(Par_3)_2 (Ar = p-FC_6H_4), H/D isotope scrambling between alkenes and IrD_2(O_2CCF_3)(Par_3)_2 was studied. No unique interpretation of the experimental data was possible, so DFT(B3PW91) calculations on the exchange process in Ir(H)_2(O_2CCF_3)(PH_3)_2(C_2H_4) were carried out to distinguish between the possibilities allowed by experiment. Of several possible mechanisms for H/D scrambling, one was strongly preferred and is therefore proposed here. It involves the insertion of the olefin to give an alkyl hydride that reductively eliminates to lead to a transition state that contains an #eta#~3-bound alkane. This transition state, which achieves a 1,1' geminal H/D exchange, is significantly lower in energy than a dihydrido carbene, located as a secondary minimum, eliminating the alternative carbene mechanism. The unexpectedly large binding energy (BDE) of the alkane ("sticky alkane") to the Ir(O_2CCF_3)(PH_3)_2 fragment (BDE = 11.9 kcal mol~(-1)) in this transition state is ascribed in part to the presence of a weakly-#sigma#- and #pi#-donating (CF_3CO_2) group trans to the alkane binding site. The H/D exchange selectivity observed requires that 1,1'-shifts (i.e., M oving to a geminal C-H bond, but not 1,3-shifts, be allowed in the alkane complex. In a key finding, a 1,3-shift in which the metal moves down the alkane chain is indeed found to have a much higher activation energy than the 1,1'-process and is therefore slow in our system. A 1,2-shift has not been considered since it would involve a strong steric hindrance at a tertiary carbon in this system. The mechanism via an alkane path provides an insight into the closely related photochemical and catalytic thermal alkane dehydrogenation processes mediated by IrH_2(O_2CCF_3)(Par_3)_2; the thermal route requires ~tBuCH=CH_2 as the hydrogen acceptor. These two alkane reactions are intimately related mechanistically to the isotope exchange because they are proposed to have the same intermediates, in particular the sticky alkane complex. Remarkably, the rate determining step of the thermal (150 deg C) alkane dehydrogenation process is predicted to be substitution of the hydrogen acceptor-derived alkane by the alkane substrate.